1. Field of the Invention
The present invention relates to a method for inspecting an intraocular lens, also called an artificial crystalline lens, which is inserted after lensectomy during cataract surgery and has the function of a convex lens, and particularly to an intraocular lens inspection method that makes it possible to judge whether or not a lens should be accepted for or rejected from use by checking for the presence and the extent of glistenings prior to clinical use.
2. Prior Art
There are conventional methods of inspecting intraocular lenses in terms of their properties, optics, sterility, etc. As discussed in xe2x80x9cGlistenings in Acrylic Lensesxe2x80x9d in the Rinsho Gankaxe2x80x94Japanese Journal of Ophthalmology, Vol. 51, No. 4, 1997, the effect on visual performance of the intraocular lenses made from a lens body of an acrylic copolymer cross-linked resin, also called acrylic lenses, used in small-incision sutureless surgery, particularly the development of glistenings seen in clinical cases having a diameter of 10 to 20 xcexcm, has recently been energetically investigated. However, an intraocular lens inspection method has not been established with which it is possible to evaluate whether or not a lens should be accepted for or rejected from use prior to clinical use.
A method of inspecting intraocular lenses prior to clinical use has not been established by conventional intraocular lens inspection methods. Therefore, there is a problem in that although the risk is known of glistenings developing due to internal structure, such as voids, etc., depending on the materials and production method used, direct use clinically without knowing the extent or status, etc., of the glistenings is unavoidable.
FIG. 5A and FIG. 5B show intraocular lens 1 made from a lens body of acrylic copolymer cross-linked resin, also called an acrylic lens, used in small incision sutureless surgery. The intraocular lens 1 is comprised of a lens body 1a and two loops 1b that hold this lens body at the posterior chamber. Lens diameter of this lens body is 5.5 to 6 mm, and maximum diameter of these two loops is approximately 13 mm. This lens body is made from an acrylic copolymer cross-linked resin having a three-dimensional structure wherein copolymer of phenylethyl acrylate and phenylethyl methacrylate is cross-linked by butanediol diacrylate. It has a reflective index of 1.55 and shows ultraviolet ray-absorbing capability. It is flexible with a Shore hardness (Shore A), which represents surface hardness, of 45. As shown in FIG. 6, it can be folded in two. With respect to thermal properties, its glass transition temperature Tg is 15.5-21.5xc2x0 C., with there being a marked increase in volume expansion at this transition temperature Tg or higher when compared to volume expansion at temperatures lower than Tg.
As shown in the histogram in FIG. 7 of the clinical glistening development period when a conventional acrylic copolymer cross-linked resin intraocular lens was used, development of glistenings during the post-surgical observation period (density of approximately 50 glistenings or more, clinical classification grade one or higher) was confirmed in 28 of 49 eyes (57%). Moreover, when the time at which the development of these glistenings was first confirmed serves as the glistening development period, there were no cases in which glistenings developed one month after surgery, but there were cases in which glistenings developed two months after surgery or later, with the greatest delay in the development of glistenings being 16 months after surgery. The average glistening development period in these 28 eyes was 6.6 months; and when clinical classification is grade 0=0 glistenings/mm3, grade 1=50 glistenings/mm3, grade 2=100 glistenings/mm3, and grade 3=200 glistenings/mm3, then the density of these glistenings covers a broad range of grade 1, grade 2, and grade 3. Today we are in the stage of energetically investigating the effect of these glistenings on visual function with respect to a variety of indicators, including visual acuity, glare, contrast, etc.
There have been no reports of cases of glistenings when conventional polymethyl methacrylate (PMMA) resin intraocular lenses have been clinically used. However, these intraocular lenses are rigid and cannot be folded in half; and therefore, there is a disadvantage in that the surgical incision is wide and they are not suitable for small-incision sutureless surgery. Furthermore, there is a double disadvantage in that the refractive index of PMMA resin is 1.49, which is smaller than the refractive index of 1.55 of the above-described acrylic copolymer cross-linked resin; and in order to obtain a lens of the same power, a thick lens of thickness d is required. Therefore, the frequency with which PMMA resin intraocular lenses are used has recently decreased.
In order to solve the above-described problems, the intraocular lens inspection method of the present invention is characterized in that an intraocular lens to be inspected is extracted randomly from an intraocular lens production lot prior to clinical use, the intraocular lens that has thus been selected is kept for a specific amount of time in physiological saline at a high-temperature and then promptly transferred to physiological saline that has been kept at a low-temperature, then the lens is examined for glistenings by being magnified and observed over time, and when a consistent number and size of glistenings are seen, it is judged that clinically analogous glistening will develop.